A man is lucky to be alive after his Apple Watch app detected the signs of a deadly blood clot.

James Green, from New York, credits the HeartWatch app on his watch for alerting him to an unusual spike in his Heart Rate.

He tweeted:

“Never thought a stupid lil wrist computer I bought two years ago would save my life.

“Saw my heart rate go up, ended up being a pulmonary embolism.”

The HeartWatch app monitors a person’s heart Rate throughout the day and notifies them when it goes above or below a certain threshold.

When James saw his heart rate go up he called his doctor.

The 28-year-old reporter from Brooklyn told the Telegraph his doctor said if he had waited any longer “it would have been fatal”.

The self-confessed “serial health tracker” explained how he used the app because he wanted to monitor data on his heart because he had previously had a blood clot on his lungs.

The app notified him that his heart rate was above his normal resting rate of 54, even when he was sitting down.

“That along with other symptoms I was having was enough data I needed to act on it, and realise it wasn’t a panic attack (since I have severe generalised anxiety), that it was something more,” he said.

James was put on blood thinners to reduce the clot when he got to hospital.